problem with today's mma training is that kids starting up with a MMA training ends up half baked. they learn striking but not enough to compete competitively in striking, either MT or boxing. Their boxing is fundamentally flawed with bad foot work and hands always down and no rotation of hips and shoulder. they learn ground game but not enough to compete competitively in BJJ. they end up jack of all trades master of none. At least for the earlier pioneers of the sport, they did master something and supplement it with other skills. We have seen NCAA champion wrestlers who added striking and submission to their arsenal, BJJ champions training in MT, etc.
Problem with today's kids is focus. malcolm gladwell wrote in his book that in order to fully master one skill you need to spend at least 10,000 hours practicing. now, if it's out of focus, imagine how many more hours you need to master BJJ, boxing and MT to be able to be competitive.
In my personal opinion, build a base then expand to other skill set.